It has been already known that a renin-angiotensin-aldosterone system participates in a rise of blood pressure (Tokyo Joshi Idar Zasshi, 60(4): 342˜350, 1999) Actually, angiotensinogen secreted from liver is converted to angiotensin I (hereinafter, referred to as AI) by the action of renin derived from kidney and is further converted to angiotensin II (hereinafter, referred to as AII) by the action of angiotensin converting enzyme (ACE). AII directly acts on blood vessel to cause vasoconstriction and also acts on adrenal cortex to promote biosynthesis and secretion of aldosterone and to cause retention of Na and water whereupon hypertension is resulted.
Therefore, as a therapeutic method for hypertension, pharmaceutical agents acting on a renin-angiotensin system have been positioned as hypotensors of the newest type at present and ACE inhibitors have been practically used. At the same time, renin inhibitors have been studied as hypotensors but, due to their side effects, their development has been given up. ACE inhibitors are specific inhibitors for angiotensin converting enzyme and captopril (generic name) is well known. However, ACE inhibitors have side effects such as dry cough and do not exhibit a clinical efficacy to the extent of an inhibitory effect on the enzymatic activity in vitro and, therefore, a combination therapy with a renin-angiotensin system inhibitor having different action mechanism has been attempted. Accordingly, there has been a strong demand for highly effective hypotensors, blood vessel thickening suppressors and organ hypertrophy suppressors which are other than the hypotensors having been reported or used already.
Renin is biosynthesized mostly in kidney as pre-prorenin comprising 406 amino acids that is a precursor thereof whose 23 amino acids at N-terminal are cleaved to give prorenin and then 43 amino acids are further cleaved from N-terminal to give renin comprising 340 amino acids. Renin is a proteinase by which angiotensinogen is specifically hydrolyzed to produce AI. However, prorenin which is a precursor of renin does not usually exhibit such an enzymatic activity. Therefore, although the amount of prorenin existing in blood is about ten-fold of that of renin, it has been believed that the active substance in a renin-angiotensin system is renin or renin that is produced by hydrolysis of prorenin.
The present inventors previously found that, when prorenin which is an inactive renin precursor forms an immune complex in vitro as it combines with an antibody which specifically recognizes a fragment of 43 amino acid (hereinafter, referred to as “profragment” or “pf”) at N-terminal which is cleaved upon at the time of production of renin from prorenin, a protein function or, in other words, an enzymatic activity (renin activity) is expressed in a non-enzymatic manner under a physiological condition without alteration in the primary structure (Japanese Patent Laid-Open No. H10-279,600 and U.S. Pat. No. 5,945,512), using an anti-human prorenin pf antibody. With regard to a means for activation of prorenin, there have been known methods of conversion to renin using protease, a method where activation is carried out without alteration in the primary structure under an acidic condition and a low-temperature (Nature, 288, 702-705, 1980), (J. Biol. Chem., 262, 2472-2477, 1987), (Clin. Chem., 37, 1811-1819, 1991), (J. Biol. Chem., 267, 11753-11759, 1992), and a method where a low-molecular renin inhibitor is combined to an enzymatically active portion buried in the deep grooves of the three-dimensional structure of prorenin (J. Biol. Chem., 267, 22837-22842, 1992) to convert into an open type, etc. However, mechanism for the activation of prorenin in vivo and its function have still been ambiguous.